Start by ticking the checkbox shown in the top right of images. This can also be done from the keyboard by focusing on the image and hitting space. When at least one image is selected, the bulk toolbar will show up at the bottom of your screen which can be focused with alt+b.

The batch actions toolbar looks like this:

From left to right, these are the provided functions in the toolbar.

Select all images

Deselect all images (which also hides the bulk toolbar)

Mark all selected images as active/visible

Mark all selected images as inactive/hidden

Search for tags to add or remove from the selected images (new in v1.8)

Remove all selected images

Keyboard users can tab through these options and execute them with space.

Batch actions are processed server-side, followed by an automatic refresh of the gallery.